The invention relates to a multiple nozzle propulsion control system generally, and more particularly to such a propulsion control system that provides high response proportional control of rocket motor jets suitable for use in aerospace vehicles such as SDI-type kinetic kill vehicles and air-to-air missiles.
While fins are currently the most commonly used method of controlling aerospace vehicles, newer systems are expected to use reaction jet controls because of potentially increased maneuverability and high altitude requirements. These systems typically comprise "bang-bang" (on-off) control schemes based on the operation of either movable poppet valves or fluidic diverter valves. Control is accomplished by driving these valves in either a thrust pulse duration modulation (PDM) or pulse width modulation (PWM) mode to achieve thrust modulation. In both cases, extremely high response valves are required. Accuracy of these systems is very dependent on valve response. In addition, the resulting thrust is in the form of high frequency pulses which have undesirable effects on the vehicle structure and electronics (e.g., guidance systems and sensors). Also, the larger these valves are, the slower the response, and correspondingly, the less accurate.
The vibrational effects discussed above will be discussed in more detail with respect to pulse width modulation and a two-nozzle system for purposes of example. One nozzle is closed, while the other is open so that a fixed thrust exits only one nozzle at any time. In this example, 100 lbs. of thrust will be used as the amount alternately exiting the nozzles. Thus, one nozzle is turned on and 100 lbs. of thrust exits in one direction; the other nozzle is off. Nozzle actuation-deactuation (on-off) can be alternated very fast. For example, one nozzle can be on for two milliseconds while the other is off. Then the other nozzle turned on for four milliseconds while the first is off. The first nozzle turned on again for two milliseconds while the second nozzle is off and those positions alternated after two milliseconds and where the second nozzle is held on for four milliseconds while the first is off and so forth. It is so fast that the body inertia of the vehicle integrates out this system thrust pulsing and it appears that only 50 lbs. thrust in one direction is applied, i.e., a thrust differential. The thrust differential can be changed by changing the time intervals discussed above. Among the drawbacks of this bang-bang (on-off) system is that the shaking or pulsing, caused by the rapidly alternating thrusts, jitters the whole body of the vehicle and adversely affects its sensors and structural integrity. That is, the constant shaking the vehicle body does generally two things. First of all, it puts a buzz into the sensors. The sensor is being jittered so the sensor loses some of its resolution. In addition, since the vehicle is being jittered at a given frequency, mechanical resonances can be put into the vehicle body and can cause structural damage. Thus, you have to design the system so that the resonance (natural frequency) of the vehicle body is different than the frequency that the pulsing system will operate. Otherwise substantial structural damage can result. Thus, there is a need to provide a highly responsive thrust control system that provides a wider range of thrust capability.